The present invention relates to a master cylinder which generates a fluid pressure for operating a brake.
As an example of a conventional master cylinder, there can be mentioned that which is disclosed in Unexamined Japanese Patent Application Public Disclosure (Kokai) No. 2-136363.
This master cylinder comprises a cylinder having one end open to the outside and the other end closed, and a secondary piston and a primary piston slidably disposed at a forward position (on a side of the closed end) and a rearward position (on a side of the open end) in the cylinder. Each piston forms a pressure chamber between itself and the cylinder, so as to generate a fluid pressure. A secondary piston guide ring, a sleeve and a primary piston guide ring are slidably fitted into the cylinder, in this order, from the open end to be provided at the forward position, a central position and the rearward position in the cylinder, respectively. A holder which abuts against the primary piston guide ring is provided at the open end of the cylinder to prevent escape of the primary piston guide ring, the sleeve and the secondary piston guide ring from the cylinder.
The secondary piston guide ring and the primary piston guide ring are adapted to axially and slidably guide the secondary piston and the primary piston. Each piston guide ring forms a fluid supply chamber between itself and an outer circumferential surface of the piston. The piston guide ring includes a communication port having one end open to the fluid supply chamber and the other end open at an outer circumferential surface of the ring. Further, the sleeve forms communication passages which allow communication between the communication ports of the secondary and primary piston guide rings and a reservoir.
However, the conventional master cylinder of Kokai No. 2-136363 has the following problems.
In this master cylinder, to prevent an excessive assembly load acting on the primary piston guide ring, the sleeve and the secondary piston, and to prevent axial play of these elements, an elastic member is provided between the secondary piston guide ring and the sleeve. However, in the case that a large force is applied in a braking operation, the sleeve moves toward the open end of the cylinder under the effect of fluid pressure generated by the primary piston, which is moved toward the closed end of the cylinder by a brake pedal. More particularly, fluid pressure works in the gap between the rear end of the secondary piston guide ring and the forward end of the sleeve where the elastic member is clamped, to thereby urge the secondary piston guide ring forwardly toward the closed end of the cylinder and the sleeve rearwardly toward the open end of the cylinder, respectively. Consequently, the play is not completely eliminated and the holder is pressed and instantaneously deformed. To prevent such deformation, the holder must be manufactured using an expensive high strength material, and be provided with a sufficient wall thickness. Accordingly, not only are costs increased but the size of the master cylinder must be increased to accommodate the extra wall thickness.
To overcome these problems, the present invention has been made. It is an object of the present invention to provide a low-cost, reduced size master cylinder in which deformation of a holder is prevented and a play of a sleeve is eliminated.
The present invention provides a master cylinder comprising a cylinder having one end open to the outside. A secondary piston guide ring, a sleeve and a primary piston guide ring are adapted to be fitted in this order into the cylinder from the one open end. A piston is slidably fitted in an axial direction of the cylinder, into each of the secondary piston guide ring, the sleeve and the primary piston guide ring. A retaining member is provided at the open end of the cylinder, and adapted to abut against the primary piston guide ring and prevent escape of the primary piston guide ring from the cylinder. An elastic member is provided between a rearwardly facing surface or rearward end portion of the sleeve and an associated forwardly facing surface or forward end portion of the primary piston guide ring.
Thus, in the present invention, the elastic member is provided between the sleeve and the primary piston guide ring. Therefore, under generation of a large force in a braking operation, unlike in the prior art, the sleeve does not play either forwardly or rearwardly. Furthermore, since fluid pressure does not create significant force between the sleeve and primary piston guide ring, the holder is not subjected to significant force and, therefore, it need not be made particularly strong.